How to truncate a given menu setting an expanded menu item as the root menu in drupal 7?

Question

In my template preprocess html and page hooks, I test the current page of being a certain kind of node, set by the menu structure. Given such a menu:

main menu

• link 1
• link 2 (specials)
• > link 2.1
• > link 2.2
• link 3

Every node which menu's link has the menu parent link 2 gets a treatment. On those nodes, I also want to only show all links inside link 2 menu, so start the main menu from that, so basically the main menu would then consist of link 2.1 and link 2.2. How would you do that?

function mytheme_preprocess_html(&$vars) {

    $special = is_special_page();

    if ($special) {
        // alter menu
    }
}

function mytheme_preprocess_page(&$vars) {

    $special = is_special_page();

    if ($special) {
        // alter menu
    }
}

function is_special_page($node) {
    $breadcrumbs = drupal_get_breadcrumb();
    $businessMatch = preg_grep("/(specials?)/i", $breadcrumbs);

    return (count($businessMatch) > 0);
}

Answer 1

I've figured it out myself, felt like sharing:

<?php
function _themename_submenu_tree_all_data($menu = 'primary-links', $mlid, $subonly = false) {
    $tree = menu_tree_all_data($menu);
    $childtree = null;

    /*
        thanks Jari http://stackoverflow.com/questions/19112294/how-to-extract-an-array-that-has-a-given-structure-and-whichs-key-value-matc/19112508?noredirect=1#comment28260688_19112508
    */

    function recursiveSearch($array, $key, $value){
      foreach($array as $sub){
        if ($sub['link'][$key] == $value)
          return $sub ;

        if (!empty($sub['below'])){
            $returned = recursiveSearch($sub['below'], $key, $value);
            if ($returned !== null)
                return $returned ;
        }
      }

        return null ;
    }

    $childtree = recursiveSearch($tree, 'mlid', $mlid);

    if ($subonly && $childtree != null) $childtree = $childtree['below'];

    return $childtree;
}

/**
 * Implements hook_preprocess_page().
 */
function themename_preprocess_page(&$vars) {
    if (arg(0) == 'node' && arg(1)) {
        $nid = arg(1);
        $node = node_load($nid);

        $vars['type'] = $node->type;

        // your internal check to see if the menu needs to be truncated

        $isbusiness = _themename_is_business_page($node);

        if ($isbusiness) {
            $trail = menu_get_active_trail();
            $mlid = $trail[ count($trail)-1 ]['mlid'];

            $sub = _themename_submenu_tree_all_data('main-menu', $mlid, true);
            $sub = menu_tree_output($sub);

            $vars['page']['main_menu']['menu_block_2']['#content'] = $sub;
        }
    }
}
?>